1. Field of the Invention
The present invention relates to a nozzle for a thermal spray gun and to a method of thermal spraying and relates particularly, but not exclusively, to a nozzle for a high velocity oxygen fuel (HVOF) thermal spray gun and method of HVOF thermal spraying.
2. Description of the Related Art
Techniques of thermal spraying, where a coating of heated or melted material is sprayed onto a surface, are well known. One such technique is high velocity oxygen fuel thermal spraying in which a powdered material, for example Tungsten Carbide Cobalt (WC—Co), is fed into a combustion gas flow produced by a spray gun and the heated particles accelerated towards a substrate that is to be coated. The powder is heated by the combustion of the fuel and oxygen mixture and accelerated through a convergent-divergent (Laval) nozzle.
Examples of HVOF thermal spray guns are disclosed in G. D. Power, E. B. Smith, T. J. Barber, L. M. Chiapetta UTRC Report No. 91-8, UTRC, East Hartford, Conn., 1991, Kamnis S and Gu S Chem. Eng. Sci. 61 5427-5439, 2006 and S. Kamnis and S. Gu Chem. Eng. Processing. 45 246-253, 2006. Nozzles from two such spray guns are shown in FIG. 1. The nozzle 10, of a HVOF spray gun, has a combustion chamber 12 into which a mixture of oxygen and fuel is injected through inlet 14 together with a powder that is to coat a substrate (not shown). Combustion of the fuel takes place in the combustion chamber and combustion gases expand and pass through a convergent-divergent restriction 16 and on through a barrel 18 before exiting through an exhaust 20.
Similarly, nozzle 22 has a combustion chamber 24 with various inlets 26 for fuel and oxygen and a convergent-divergent nozzle 28 with an extended divergent portion forming a barrel which contains an exhaust 30. The powder coating is introduced into the barrel as the divergence begins.
To avoid oxidation of the powdered material, heating must take place smoothly over a range of temperatures without exceeding a critical value. The temperature at which oxidation starts for most sprayed materials is well below the maximum flame temperature of around 3300K. For example, Tungsten Carbide Cobalt oxidation starts at a surface temperature of around 1500K. As a result, injection of the powder into the centre of the combustion chamber is not appropriate for this material and generally for non-ceramic materials and therefore the powdered material must be injected into the stream of supersonic gases. However, this gives the particles momentum in a radial direction making them likely to leave the gas stream before impacting on the article to be coated. Furthermore, bigger and heavier particles follow different trajectories compared to smaller, lighter ones. In practice, particle spreading reduces the spraying accuracy and decreases deposition efficiency because particle impact is not normal to the surface that is being coated.
Furthermore, injection of the powder into the nozzle results in damage to the nozzle, in particular erosion of the barrel's wall, and as a result the nozzle, or at least the barrel section, typically must be replaced every ten hours of operation.
When the rate of flow of combusted gases and powder particles accelerates to supersonic velocities, a series of expansion and compressions take place within the barrel. The gas stream in the interior expands and cools and is compressed and heats as it passes through the shock diamonds. The shock wave diamonds result in a loss of temperature and expansion on exiting the barrel increases the temperature loss. An overall decrease in static temperature (from around 3000K to around 2000K) and an overall increase in velocity (from around 200 m/s to around 1800 m/s) after compression and expansion in the convergent-divergent nozzle region, produces this behaviour inside the barrel. When the powder is injected into the high velocity gas stream, its dwell time is decreased due to an increased rate of acceleration. Therefore to ensure sufficient particle heating, a long barrel is required to maintain high gas temperatures. This long barrel, typically 350 mm, limits the applications to which the thermal sprayer can be applied, for example, internal surfaces of even quite large components are impossible to spray.
Small particles, below 10 μm, cannot practically be used because such small powdered material disperses in the gas field and consequently rebound from or never reach the article being sprayed. As a result, the small particles never reach the flow centre line and therefore cannot benefit from the high velocity/temperature flow regions. Instead they follow a route on the border of the free jet and when mixing with the ambient air outside the barrel starts, they diffuse in all directions. The lightweight particles are therefore chasing the flow direction and consequently are blown away from the substrate.
Preferred embodiments of the present invention seek to overcome the above described disadvantages of the prior art.